Yes, it is great to be sure of all the cases you can imagine. It is really important to have a clear overview of everything can happen. Right after that, you have an important task, which is to balance completeness with speed. Obviously, 100% is awesome and you should aim for it. However, know which ones won’t matter because they won’t be used. Alternative ideas:

• Make the feature less complicated to eliminate some of the flows
• Add a non-functioning button which triggers something at your end to know if someone ends up in the unbuilt flow
• Have events in place to know exactly how often the flow would be used
• Consider doing some stuff by hand or delegating things to support

The important thing is to know which cases you’re not supporting. If you don’t know what’s in your last 1 percent, you cannot be sure you’re on 99.